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The variation of Rayleigh ellipticity versus frequency is gaining popularity in site characterization. It becomes a necessary observable to complement dispersion curves when inverting shear wave velocity profiles. Various methods have been proposed so far to extract polarization from ambient vibrations recorded on a single three-component station or with an array of three-component sensors. If only absolute values were recovered 10 yr ago, new array-based techniques were recently proposed with enhanced efficiencies providing also the ellipticity sign. With array processing, higher-order modes are often detected even in the ellipticity domain. We suggest to explore the properties of a high-resolution beamforming where radial and vertical components are explicitly included. If N is the number of three-component sensors, 2N x 2N cross-spectral density matrices are calculated for all presumed directions of propagation. They are built with N radial and N vertical channels. As a first approach, steering vectors are designed to fit with Rayleigh wave properties: the phase shift between radial and vertical components is either -Pi/2 or Pi/2. We show that neglecting the ellipticity tilt due to attenuation has only minor effects on the results. Additionally, we prove analytically that it is possible to retrieve the ellipticity value from the usual maximization of the high-resolution beam power. The method is tested on synthetic data sets and on experimental data. Both are reference sites already analysed by several authors. A detailed comparison with previous results on these cases is provided.

In this study data and results of a high-resolution experiment in Cephalonia (Greece) regarding empirical basin effects are presented. A total of 59 velocimeters and 17 accelerometers were deployed in the basin of Argostoli Cephalonia (Greece), for a period of 7 months (September 2011–April 2012). Due to high seismicity of the western Greece and surrounding area this array recorded thousands of local, regional and global events. Data used in this work come from a selection of 162 regional and local earthquakes, 3 km ≤ R ≤ 600 km, with magnitude range, 1.0 ≤ M ≤ 5.2. Based on high signal-to-noise ratio recordings and two selected reference stations, variation of several intensity measures (PGA, PGV, Arias Intensity, Cumulative Absolute Velocity), significant duration, HVSR and SSR of ground motion recordings on soil sites within the basin is carefully examined for a range of frequencies of engineering interest. Comparison of results with a detailed 2D geologic model shows a good consistency both in amplification and frequency domain. Influence of “reference” site on ground motion variation of soil sites is also discussed in light of our results. Finally, it is suggested that 2D or/and 3D theoretical modeling should be performed given the availability of geological and geophysical parameters to define a realistic model of the basin. Results of this study can undoubtedly serve in model validation and improvement of ground motion simulation tools.

International audience; Data provided by accelerometric networks are important for seismic hazard assessment. They are key to derive Ground Motion Prediction Equations (GMPEs). The correct use of accelerometric signal is also linked to the station site metadata that include reliable information about site class (VS30), velocity profiles, and other relevant information that can help to quantify the site effect associated to stations. In France, the permanent accelerometric network consists of about 150 stations. A recent project led to the characterization of around 30 stations, especially in South East of France. This characterization project was performed using surface-wave based methods that allow the derivation of velocity profiles from dispersion curves of Rayleigh and Love waves. We implemented both active acquisitions (Multichannel Analysis of Surface Waves) along lines from 50 to 100 m length and passive acquisitions (Ambient Vibration Array) using multiple circle arrays (apertures from 10 to 1000 m). The computation of dispersion curves, then their inversion in terms of shear wave velocity profiles (taking into account the non-uniqueness issue of such inversion) allowed the estimations of VS30 values and the designation of soil classes including the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) Perform both active and passive measurements in order to derive dispersion curves for an adequate frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities, which reduces the risk of misattribution of modes and thus, mitigates errors when modeling velocity profiles. Even when logistical contexts are sometimes difficult, the use of surface-wave based methods are suitable for station site characterizations, even on rock sites (where the applicability of these methods was sometimes disputed). Typically, it is possible to achieve a complete survey for one station in one working day, by 5 to 6 motivated operators. Conversely, the processing is time consuming (one working week for one geophysicist) and the inversion procedure has to be supervised by an expert in surface wave methods.

International audience; The InterPacific project (Intercomparison of methods for site parameter and velocity profile characterization) aims to assess the reliability of seismic site characterization methods (borehole and surface wave methods) used for estimating shear wave velocity (VS) profiles and other related parameters (e.g., VS30). Three sites, representative of different geological conditions relevant for the evaluation of seismic site response effects, have been selected: (1) a hard rock outcrop, (2) a deep soft deposit, and (3) an intermediate case with thick stiff soils and a velocity inversion and large bedrock depth. Two to three boreholes have been drilled at these sites and various companies were invited to perform in-hole measurements (cross-hole, down-hole and PS-logging). Both active and passive surface wave data were also collected. All of them located in the vicinity of the boreholes to facilitate the robust comparison between results from the invasive and non-invasive methods. The same experimental non-invasive datasets, without any prior information about the sites, were provided to the different teams, which were asked to retrieve the Vs profiles while working on the preferred subset of available experimental data. For surface wave methods, results indicate that the dispersion curves provided by the participants were in very good agreement with each other. Inverted Vs profiles were also found to be very consistent with respect to the known reliable resolution depth ranges of these techniques. Results obtained from invasive methods show a variability of VS estimates on the same order as about the variability of Vs estimates obtained from non-invasive methods. We also delivered guidelines to recommend good practices for non-expert users. These guidelines provide practical indications on the acquisition and analysis of surface wave data by reviewing basic principles and providing specific suggestions related to common situations. The guidelines are primarily targeted to non-expert users applying surface wave testing techniques, but can be useful to specialists in the field as a general reference guide. These guidelines, however, cannot be a substitute for experience in surface wave analyses; rather, they provide a common reference to establish the necessary dialogue between the service provider and the end-user of the results. The guidelines are based on the experience gained within the InterPacific project and on the expertise of the participants in acquisition and analysis of surface wave data.

International audience; It has been observed repeatedly in the post-earthquake investigations that buildings having frequency similar to soil frequency (coming from H/V for example) exhibit significantly greater damage due to the double resonator concept (Caracas 1967, Mexico 1985, L'Aquila 2009). However this observation is generally not taken directly into account neither in present-day seismic regulations (small scale), nor in large-scale seismic risk analysis. We considered a theoretical analysis to study the effect of frequency coincidence between soil and building. As a first step, 887 natural soil profiles with linear behavior are associated to a set of single degree of freedom elastoplastic oscillators. The results obtained are used to quantify the damage increment related to the soil-building frequency coincidence and depending on different parameters such as the loading level characterized by the peak ground acceleration (PGA), the soil profile (impedance contrast, soil frequency) and the building (ductility, fundamental frequency). This statistical work is based on Artificial Neural Network (ANN) approach that does not require any prior knowledge, confirming that the main parameter controlling the damage increase is the ratio structure frequency to soil frequency (fstruct/fsoil), with a synaptic weight exceeding 58% (when PGA represents 27.05%, the impedance contrast 10.44% and ductility 4.24%). The leading parameter, i.e. the fstruct/fsoil ratio, controls also the damage increment when considering various ductility classes with a synaptic weight percentage of 45%; the parameter that follows is the PGA.

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data., Bulletin of Earthquake Engineering, 16 (6), ISSN:1570-761X, ISSN:1573-1456